1. Field of the Invention
This invention relates to a fixing-unit roller used in fixing units used in electrophotographic image-forming apparatus such as copying machines and LBPs (laser beam printers) and is required to have both toner releasability and scratch/wear resistance, and also relates to a process for its production and a fixing assembly employing such a fixing-unit roller.
2. Related Background Art
In electrophotographic apparatus, used as a pair of rollers with which toner images are fixed by heat and pressure, i.e., a fixing roller and a pressure roller, are those comprising a cylindrical mandrel and formed thereon an elastic material layer having a single-layer or multi-layer structure. As materials for forming the elastic material layer, organopolysiloxanes (silicone rubbers) and fluoroelastomers are used, which have good heat resistance, chemical resistance and weatherability.
Important properties required when these materials are used as elastic materials for the outermost layer in the fixing or pressure roller are chiefly exemplified by toner releasability and scratch/wear resistance.
The toner releasability is meant to be an unlikelihood for the toner to adhere to the roller surface when a toner image formed electrostatically is fixed by heat and pressure between fixing and pressing rollers. When the material used in the outermost layer of the roller has a poor toner releasability, the toner may adhere to the roller surface to cause toner offset, where blank areas appear in copied images. Also, even when a material having relatively so good a toner releasability as to cause no toner offset, the fixing of toner images repeatedly may make the toner releasability poor to cause the toner offset in some cases at a stage where copies have been taken on hundreds or thousands of sheets. Accordingly, in view of the performance of the roller, it is important to use in the outermost layer a material having a good toner releasability.
The scratch/wear resistance is meant to be an unlikelihood for the roller surface to become scratched as a result of the repeated fixing of toner images.
As a member set in contact with the fixing roller or pressure roller, in the case of the fixing roller for example, a web for applying silicone oil and for cleaning its surface to remove the offset toner is set in contact with it in some instances in order to prevent the toner offset from occurring. In such a case, as a result of the repeated fixing of toner images, the offset toner or paper dust may adhere to the part where the web is in contact, and such matter having adhered may cause scratch lines on the roller surface. Also, in the case of the pressure roller, for example, a blade for scraping off any excess silicone oil is set in pressure contact with the roller surface in some instances. In such a case, the offset toner or paper dust may likewise adhere to the part where the blade is in pressure contact, and such matter having adhered may cause scratch lines on the roller surface. Where the scratch lines have occurred on the roller surface, in the case of the fixing roller, the scratch lines may adversely affect copied images. In the case of the pressure roller, the offset toner may gathers at the part of scratch lines to cause contamination due to the toner adhering to the back of a recording material such as paper.
When a material having poor scratch resistance and wear resistance is used in the outermost layer, the roller surface tends to be scratched to cause the problems as stated above. Accordingly, it is also important to use in the outermost layer a material having good scratch resistance and wear resistance.
Materials' physical properties that determine the toner releasability or the scratch/wear resistance are chiefly exemplified by surface energy and hardness. It is thought that, the lower surface energy a material has, the better the toner releasability is. It is also thought that, the higher hardness a material has, the better the scratch/wear resistance is. Organopolysiloxanes, which are relatively low in both surface energy and hardness, are considered to be superior in toner releasability but inferior in scratch/wear resistance. On the other hand, fluoroelastomers are relatively high in both surface energy and hardness, and, contrary to the organopolysiloxanes, are considered to be superior in scratch/wear resistance but inferior in toner releasability.
In the past, for the purpose of improving the scratch/wear resistance of organopolysiloxanes, proposed are an organopolysiloxane incorporated with fused silica having an average particle diameter of 0.5 to 20 .mu.m (Japanese Patent Application Laid-open No. 8-193166) and an organopolysiloxane incorporated with spherical tetrafluoroethylene resin particles (Japanese Patent Application Laid-open No. 9-12893). These conventional materials, however, are both organopolysiloxanes mixed with resin, and it follows that the properties of organopolysiloxanes as elastic materials are damaged to a certain extent. Hence, in order to obtain composite materials having good properties as elastic materials, it is considered effective to incorporate organopolysiloxanes not with resin but with an elastic material having good scratch/wear resistance. One of such materials may include fluoroelastomers.
Accordingly, in order to obtain materials having good physical properties inherent individually in organopolysiloxanes and fluoroelastomers, studies have hitherto been made on composite materials prepared by mixing the both. For example, proposed are a roller comprising an aluminum mandrel and provided around it an elastic material layer formed using a composite material composed of an organopolysiloxane and a fluoroelastomer which have been mixed mechanically or by dissolving or dispersing them in an organic solvent (Japanese Patent Application Laid-open No. 5-147126), those making use of a composite material having a fluoroelastomer as a matrix component and, dispersed in this fluoroelastomer matrix, particles comprised of at least one of a reactive silicone oil and a reactive fluorine oil (Japanese Patent Application Laid-open No. 9-96981), and a fixing member making use of a uniform-system composite material obtained by adding an amine coupler having a silane side group and a hydrolyzable silane compound such as tetraethoxysilane to cause a fluoroelastomer and an organopolysiloxane to combine chemically (Japanese Patent Application Laid-open No. 6-308848).
In the prior art disclosed in Japanese Patent Application Laid-open No. 5-147126, a method of mixing materials mechanically is proposed as a means for preparing the composite material. However, the state of dispersion that is attained by mixing materials mechanically is relatively rough, because organopolysiloxanes commonly have a low viscosity and fluoroelastomers a low Mooney viscosity and their difference in viscosity is too great for them to be dispersible with ease. Thus, an island phase comprised of a fluoroelastomer may have particle diameters of tens to hundreds of micrometers. Taking account concurrently of the fact that toners have particle diameters of several micrometers, it can be said that any satisfactory state of dispersion has not been attained. Also, in the same prior art disclosed in Japanese Patent Application Laid-open No 5-147126, as another means for preparing the composite material, it is proposed to dissolve or disperse materials in an organic solvent. However, composite materials obtained by mixing dissolving or dispersing materials in an organic solvent commonly have a great difference in polarity between the organopolysiloxane and the fluoroelastomer, where the both are poorly compatible with each other. Hence, depending on the rate of evaporation of the solvent, either of them may be present in the matrix in the form of large agglomerates of about tens to hundreds micrometers in size. Thus, their state of dispersion is likewise rough in some cases. Moreover, in full-color machines more required to have toner releasability than monochromatic machines, there is a problem that the roller surface has a little insufficient toner releasability because the island phase fluorine rubber has a high surface energy.
In the prior art disclosed in Japanese Patent Application Laid-open No. 9-96981, taking account of such problems, particles comprised of a reactive silicone oil or a reactive fluorine oil are made to partly react and fix in the fluoroelastomer matrix so that they are dispersible in a state of particle diameters of 30 .mu.m or smaller. The composite material thus obtained is improved in the state of dispersion. However, the fluoroelastomer, one having a higher surface energy than the other, stands as a sea phase. Hence, compared with a composite material having a sea phase and an island phase in a fashion opposite to this prior art, i.e., a composite material in which the organopolysiloxane stands as a sea phase, the latter is considered advantageous from the viewpoint of toner releasability.
In the prior art disclosed in Japanese Patent Application Laid-open No. 6-308848, it is described that a uniform-system composite material comprised of a organopolysiloxane and a fluoroelastomer is obtainable in which they have been united at the level of polymer molecules. However, it does not disclose any form of composite material that is a dispersion system having a island-in-sea structure wherein the organopolysiloxane stands as a sea phase and the fluoroelastomer as an island phase.